A picture of the damage would be helpful. Vine weevil beetles are partial to munching on rhododendron foliage. Is the damage around the edges or in the middle of the leaves?

My first thought, too. Notching at the leaf edges is vine weevil. Big chunks all down the side is usually snail damage. One small picture would confirm, and then comes the remedy. W

At this time of year I'd lay odds it was vine weevils.

Vine weavil are the traditional enemy of rhododendrons and take notches out of the edges of the leaves. They come out at night so you could try going round with a torch and collecting them, otherwise there is not a lot you can do in the garden to prevent this.

my bet also is vine weavils ,there are remindies out there in you local garden centers to treat these.

If it is not notch damage and more bite marks, then caterpillars/sawflies larvae. Certainly Sawfly larvae have been munching on my azalea.

As Bulbaholic says, little you can do in the garden, bar do a night hunt for the adults (all females) who quickly drop onto the ground, and play dead, upside down. However, in pots it's easier, and as Bluemoon says, a certain named remedy is effective against the larvae, and therefore future problems.